In complementary bipolar process the process specifics do not allow conventional lateral ESD protection devices to be formed.
Furthermore, difficulties are faced by bipolar junction devices as electrostatic discharge (ESD) protection structures, since in the case of high performance radio frequency bipolar junction transistors (RF BJTs) the subcollector is typically heavily doped which results in a high holding voltage and low on-state resistance due to the high density of injected carriers required after snapback. Such a prior art vertical BJT is shown in cross section in FIG. 1, which shows an n-polysilicon emitter 100 formed on a p-base 102 and an n-epitaxially grown collector 104 with subcollector in the form of an n-buried layer (NBL) 106 contacted through an n-sinker 108.
SCRs as ESD protection structures also face difficulties in a bipolar process since in a vertical bipolar process the collector of an npn BJT is typically implemented using a subcollector comprising an n-buried layer. This n-buried layer poses difficulties for SCR devices since it creates a high level of isolation of the p-emitter of the SCR. One prior art solution has been to partially block the subcollector. This is shown in FIG. 2, which shows a cross-section through an SCR. The SCR includes the npn structure of a vertical bipolar transistor comprising emitter 200, p-base 202, and collector 204 with subcollector in the form of NBL 206 contacted through n-sinker 208. In addition it includes a p-emitter 210 in the form of a sinker to define an npnp structure. As is shown in FIG. 2, the collector is partially blocked by a shallow trench isolation (STI) region 212.
The present invention seeks to provide an alternative ESD protection structure in the case of a complementary silicon-germanium (SiGe) bipolar process.